The present invention relates to semiconductor devices and more particularly to protective surface treatment of aluminum or aluminum alloy interconnection layers to avoid undesired corrosion. At this time, most semiconductor devices employ aluminum or aluminum alloy members as the conductive conduit on the chip circuit. Aluminum possesses several desired chemical and physical properties which make it an attractive choice for this use. These properties non-exclusively include low resistivity, ease of deposition onto semiconductor wafer components and good adhesion onto silicon dioxide. However, unfortunately, thin aluminum films are easily corroded in the presence of even small amounts of moisture and a small amount of an ionic species, especially chlorides. In order to prevent the corrosion of the aluminum interconnections on semiconductors, manufacturers usually apply a coating of glass or nitride over the top of the aluminum. This prevents the corrosion of the aluminum except for a small area necessary to connect the interconnect layer to the external package pins of the semiconductor. Unfortunately, this area is especially susceptible to corrosion. Moisture has been shown to travel along the interface between the package leads and the plastic packaging material directly to the exposed aluminum at the point of connection. This corrosion is the primary cause of failure for plastic packaged semiconductors in a moist environment.
Attempts by others to solve this problem require either an organic moisture barrier, or a multilayer metal process. Many organic coatings have been suggested as a moisture barrier including epoxies, silicones and parylene. None have been found to be effective. All show some moisture permeability through the material, and all allow migration between the package leads and the coating at their interface.
Multilayer metal treatments have been found to be an effective technique for preventing the corrosion of the aluminum. These techniques apply a thin barrier such as titanium or tungsten over the exposed aluminum, then coat this with a noble metal such as gold. The drawbacks to this technique include the expense of the complicated process of applying and patterning these additional layers. In addition, noble metals are susceptible to dendritic formation in moist environments. Dendritic formation may be as destructive to the microcircuit as aluminum corrosion. The present invention provides an improved process which renders such exposed aluminum much less susceptible to such corrosion. In this regard, the invention relates to the use of zinc chromate to passivate the surface of the aluminum which is exposed to a corrosive environment, especially at the contact pads of a semiconductor element. This zinc chromate treatment is applied to aluminum without the use of organic binders to hold the chromate to the aluminum surface.